Global MHD Simulations of Neptune's Magnetosphere

A global magnetohydrodynamic (MHD) simulation has been performed in order to investigate the outer boundaries of Neptune's magnetosphere at the time of Voyager 2's flyby in 1989 and to better understand the dynamics of magnetospheres formed by highly inclined planetary dipoles. Using the MHD code Gorgon, we have implemented a precessing dipole to mimic Neptune's tilted magnetic field and rotation axes. By using the solar wind parameters measured by Voyager 2, the simulation is verified by finding good agreement with Voyager 2 magnetometer observations. Overall, there is a large-scale reconfiguration of magnetic topology and plasma distribution. During the “pole-on” magnetospheric configuration, there only exists one tail current sheet, contained between a rarefied lobe region which extends outward from the dayside cusp, and a lobe region attached to the nightside cusp. It is found that the tail current always closes to the magnetopause current system, rather than closing in on itself, as suggested by other models. The bow shock position and shape is found to be dependent on Neptune's daily rotation, with maximum standoff being during the pole-on case. Reconnection is found on the magnetopause but is highly modulated by the interplanetary magnetic field (IMF) and time of day, turning “off” and “on” when the magnetic shear between the IMF and planetary fields is large enough. The simulation shows that the most likely location for reconnection to occur during Voyager 2's flyby was far from the spacecraft trajectory, which may explain the relative lack of associated signatures in the observations

The heliospheric magnetic field

The heliospheric magnetic field (HMF) is the extension of the coronal magnetic field carried out into the solar system by the solar wind. It is the means by which the Sun interacts with planetary magnetospheres and channels charged particles propagating through the heliosphere. As the HMF remains rooted at the solar photosphere as the Sun rotates, the large-scale HMF traces out an Archimedean spiral. This pattern is distorted by the interaction of fast and slow solar wind streams, as well as the interplanetary manifestations of transient solar eruptions called coronal mass ejections. On the smaller scale, the HMF exhibits an array of waves, discontinuities, and turbulence, which give hints to the solar wind formation process. This review aims to summarise observations and theory of the small- and large-scale structure of the HMF. Solar-cycle and cycle-to-cycle evolution of the HMF is discussed in terms of recent spacecraft observations and pre-spaceage proxies for the HMF in geomagnetic and galactic cosmic ray records

Measurements of Suprathermal Particles at 1 AU and in the inner Heliosphere

In this thesis, interplanetary suprathermal particles at 1 astronomical unit (AU) are studied with three time-of-flight mass spectrometers onboard three spacecraft. We study the variation and evolution of suprathermal particles during stream interaction regions (SIRs). As preparation work, the background of the Suprathermal Time-of-Flight spectrometer (STOF) of the Solar and Heliospheric Observatory (SOHO) is analyzed. We identify that the STOF background is mainly caused by energetic penetrating particles. Due to a possible leakage of photons at the entrance system of STOF, these particles are more easily recorded by STOF than originally anticipated. In addition, we propose a method for the background estimation. This part of work guides the event selection for the SIR analysis. In addition, based on this work, we further estimate the background for the Suprathermal Electrons and Protons sensor (STEP) which will be carried by the Solar Orbiter spacecraft and start its journey of exploration in 2020. Both STOF and STEP cover suprathermal energies and the main source of their background is energetic penetrating particles. For the SIR study, we have observed that the time profile of the suprathermal particles peaks inside the compressed fast wind (F') region, close to the trailing edge. When observers travel from the F' region via the trailing edge into the undisturbed fast wind (F) region, spectra harden with time, together with an increase of the He+/He++ abundance ratio. These observations are consistent with previous ones, but cover lower suprathermal energies than before. Moreover, we have identified turnover spectra at low suprathermal energies during some SIR events, excluding the instrumental influence, e.g., efficiency and background. The turnover spectral shape is predicted by the classical Fisk & Lee theory but has not been reported in previous observations so far

Earth’s climate response to a changing Sun

For centuries, scientists have been fascinated by the role of the Sun in the Earth’s climate system. Recent discoveries, outlined in this book, have gradually unveiled a complex picture, in which our variable Sun a¬ffects the climate variability via a number of subtle pathways, the implications of which are only now becoming clear. This handbook provides the scientifically curious, from undergraduate students to policy makers with a complete and accessible panorama of our present understanding of the Sun-climate connection. 61 experts from di¬fferent communities have contributed to it, which reflects the highly multidisciplinary nature of this topic. The handbook is organised as a mosaic of short chapters, each of which addresses a specific aspect, and can be read independently. The reader will learn about the assumptions, the data, the models, and the unknowns behind each mechanism by which solar variability may impact climate variability. None of these mechanisms can adequately explain global warming observed since the 1950s. However, several of them do impact climate variability, in particular on a regional level. This handbook aims at addressing these issues in a factual way, and thereby challenge the reader to sharpen his/her critical thinking in a debate that is frequently distorted by unfounded claims